The present invention relates to a miniature multistage rotary switch with a metallic part of the switch casing (case member) consisting of an injection-moulded part, with a threaded bushing moulded thereto serving as the bearing of the switch spindle and for mounting the switch to a board, provided with an interior ball-rest unit co-operating with detent elements pressing radially towards the outside and arranged inside a rotor connected to the switch spindle, and which resiliently press against detent cams of the ball-rest unit.
There are miniature multistage rotary switches already known in the art whose casing together with the threaded bushing moulded thereto, are made from an injection-moulded (die-cast) zinc. On its interior side wall, the casing is provided with detent cams moulded thereto, which co-operate with spring-loaded steel balls. Such types of switches have to withstand a great number of switching cycles. For this purpose, the detent path is provided with a suitable lubricant. In spite of this, such a type of switch can only be used within a restricted temperature range, because the number of e.g. 20,000 switching cycles which are performable at room temperature, drops to about 1,000 to 2,000, and in the utmost to 5,000 at a temperature of 85.degree. C.
Furthermore, this temperature range is restricted owing to the fact that the lubricant, at lower temperatures, has a higher viscosity, so that the switch becomes more and more difficult to turn. To this there is still to be added that the lubricant may easily enter the contact path. Of course, the contact path must likewise be slightly lubricated, but the detent lubricant is unsuitable for the use with contacts, so that by this, the service life of the contents is strongly affected.
Moreover, with respect to multistage rotary switches it is already known for the switch casing together with the threaded bushing moulded thereto, to be made from a suitable plastics material which has good anti-friction properties, i.e. a low coefficient of friction. Therefore, a separate lubrication of the detent path is not necessary. These types of switches were not found to have a reduced number of switching cycles at increased temperatures. As a disadvantage of this type of switches, however, there is considered the smaller mechanical strength, in particular of the threaded bushing. This bushing, therefore, especially at increased temperatures, cannot be loaded so strongly, so that the built-in switch is likely to become loose.
Moreover, in the case of an excessively high starting torque, the threaded bushing is likely to become deformed, so that the switch spindle no longer runs smoothly or becomes no longer rotatable at all.
It is an object of the present invention to solve the problem of designing miniature multistage rotary switches in such a way that they, by maintaining a high mechanical strength, are fully capable of performing their operational functions throughout a wide temperature range.